Takes a Super Metroid sprite sheet and quarters each frame to create a broken looking sprite

sprite_splitter.lua

gamelicense = "Compatible"
-- replace subcritical with some library that can be used to read/write a png.
require "subcritical"
sc.no_strict_path = true

-- replace pngload and pngdump with png read and write functions.
local pngload = assert(SC.Construct("PNGLoader"))
local pngdump = assert(SC.Construct("PNGDumper"))

local passed_path= ...
local img_path= scpath(passed_path, 3)
local source= assert(pngload:Load(img_path))

local src_w, src_h= source:GetSize()

-- Construct a sprite sheet the same size as the source to copy to.
local dest= sc.Construct("Graphic", src_w, src_h)

local function split_frame(x, y, w, h)
	local hw = w * .5
	local hh = h * .5
	local mx = x + hw
	local my = y + hh
  -- replace dest:Copy with whatever copies from one sprite sheet to another
	dest:Copy(source, x, y, hw, hh, mx, my)
	dest:Copy(source, mx, y, hw, hh, x, my)
	dest:Copy(source, x, my, hw, hh, mx, y)
	dest:Copy(source, mx, my, hw, hh, x, y)
end

local oopsed = false

local function is_delimiter(r, g, b, a)
	-- 0, 0, 127 is the frame around all sprites
  -- is_delimiter returns 1 or 0 for some bit math later.
	return (a >= .5 and r == 0 and g == 0 and b >= .48 and b <= .52) and 1 or 0
end

-- Each corner is recognized by its pattern of four bits.
local corner_names = {
	-- 10
	-- 00
	[1] = 'inner_upper_left',
	-- 01
	-- 00
	[2] = 'inner_upper_right',
	-- 00
	-- 10
	[4] = 'inner_lower_left',
	-- 11
	-- 10
	[7] = 'upper_left',
	-- 00
	-- 01
	[8] = 'inner_lower_right',
	-- 11
	-- 01
	[11] = 'upper_right',
	-- 10
	-- 11
	[13] = 'lower_left',
	-- 01
	-- 11
	[14] = 'lower_right',
}

-- short corner names are for recognizing shapes of frames.
local short_ctypes = {
	['inner_upper_left'] = 'q',
	['inner_upper_right'] = 'e',
	['inner_lower_left'] = 'z',
	['upper_left'] = 'Q',
	['inner_lower_right'] = 'c',
	['upper_right'] = 'E',
	['lower_left'] = 'Z',
	['lower_right'] = 'C',
}

local function get_corner_type(x, y)
  -- replace GetSRGBPixel with whatever reads the r,g,b,a values from a single pixel of the png.
  -- Multiplies the 0 or 1 returned by is_delimiter for the bit math mentioned earlier.
	local ulbit = is_delimiter(source:GetSRGBPixel(x, y))
	local urbit = is_delimiter(source:GetSRGBPixel(x+1, y))*2
	local llbit = is_delimiter(source:GetSRGBPixel(x, y+1))*4
	local lrbit = is_delimiter(source:GetSRGBPixel(x+1, y+1))*8
	local sum = ulbit + urbit + llbit + lrbit
	return sum
end

-- used_corners will make sure that we don't process a frame twice when it has two upleft corners.
local used_corners = {}

-- scan functions find the outline of a frame.
local scan_left
local scan_right
local scan_up
local scan_down

scan_left = function(sx, sy)
	for x = sx, 0, -1 do
		local ctype = corner_names[get_corner_type(x, sy)]
		if ctype == 'lower_left' then
			return x, sy, scan_up, ctype
		elseif ctype == 'inner_lower_right' then
			return x, sy, scan_down, ctype
		end
	end
	return nil, nil, 'ran off'
end

scan_right = function(sx, sy)
	for x = sx, src_w-2 do
		local ctype = corner_names[get_corner_type(x, sy)]
		if ctype == 'upper_right' then
			return x, sy, scan_down, ctype
		elseif ctype == 'inner_upper_left' then
			return x, sy, scan_up, ctype
		end
	end
	return nil, nil, 'ran off'
end

scan_up = function(sx, sy)
	for y = sy, 0, -1 do
		local ctype = corner_names[get_corner_type(sx, y)]
		if ctype == 'upper_left' then
			return sx, y, scan_right, ctype
		elseif ctype == 'inner_lower_left' then
			return sx, y, scan_left, ctype
		end
	end
	return nil, nil, 'ran off'
end

scan_down = function(sx, sy)
	for y = sy, src_h-2 do
		local ctype = corner_names[get_corner_type(sx, y)]
		if ctype == 'lower_right' then
			return sx, y, scan_left, ctype
		elseif ctype == 'inner_upper_right' then
			return sx, y, scan_right, ctype
		end
	end
	return nil, nil, 'ran off'
end

-- func_names is just an unused lookup in case I needed to debug handle_frame getting into a weird state.
local func_names = {
	[scan_left] = 'scan_left',
	[scan_right] = 'scan_right',
	[scan_up] = 'scan_up',
	[scan_down] = 'scan_down',
}

-- handle_frame is called when an upper left corner is found.
-- needs to find the outline of the frame and split it appropriately.
local function handle_frame(left, top)
	local corners = {{left, top, 'upper_left'}}
	local state = scan_right
	local x, y = left, top
	local corcode = 'Q'
	while true do
		used_corners[x+(y*src_w)] = true
		local nc_x, nc_y, next_state, ctype = state(x, y)
		if next_state == 'ran off' then
			return
		end
		if nc_x == left and nc_y == top then
			break
		else
			corcode = corcode .. short_ctypes[ctype]
			corners[#corners+1] = {nc_x, nc_y, ctype}
			x = nc_x
			y = nc_y
			state = next_state
		end
	end
	local l
	local t
	local r
	local b
	if corcode == 'QECZ' then
    -- typical rectangle
		l = corners[1][1]
		t = corners[1][2]
		r = corners[3][1]
		b = corners[3][2]
		-- 1111 1111
		-- 1011 1001
		-- 1001 1011
		-- 1111 1111
	elseif corcode == 'QEeECZ' then
    -- Some angle aiming frames.
		l = corners[1][1]
		t = corners[3][2]
		r = corners[4][1]
		b = corners[5][2]
	elseif corcode == 'QECcCZ' then
    -- Some angle aiming frames.
		l = corners[1][1]
		t = corners[1][2]
		r = corners[5][1]
		b = corners[5][2]
		-- 1111 1111
		-- 1001 1101
		-- 1101 1001
		-- 1111 1111
	elseif corcode == 'QECZzZ' then
    -- Some angle aiming frames.
		l = corners[4][1]
		t = corners[2][2]
		r = corners[2][1]
		b = corners[4][2]
	elseif corcode == 'QECZQq' then
    -- Some angle aiming frames.
		l = corners[5][1]
		t = corners[5][2]
		r = corners[3][1]
		b = corners[3][2]
		-- 11111 11111 11111
		-- 11011 11001 11001
		-- 10001 10001 10001
		-- 10001 10011 11001
		-- 11111 11111 11111
	elseif corcode == 'QEeECZQq' then
		print('unused octrect '..left..','..top..': '..corcode)
	elseif corcode == 'QECcCZQq' then
		-- used in grapple
		if corners[5][2] - corners[1][2] > corners[3][1] - corners[7][1] then
			l = corners[1][1]
			t = corners[1][2]
			r = corners[5][1]
			b = corners[5][2]
		else
			l = corners[7][1]
			t = corners[7][2]
			r = corners[3][1]
			b = corners[3][2]
		end
	elseif corcode == 'QECZzZQq' then
			l = corners[1][1]
			t = corners[1][2]
			r = corners[3][1]
			b = corners[3][2]
		-- 11111 11111 11111
		-- 10011 10011 10001
		-- 10001 10001 10001
		-- 10011 11001 11011
		-- 11111 11111 11111
	elseif corcode == 'QEeECcCZ' then
			l = corners[1][1]
			t = corners[1][2]
			r = corners[7][1]
			b = corners[7][2]
	elseif corcode == 'QEeECZzZ' then
		if corners[6][2] - corners[2][2] > corners[4][1] - corners[8][1] then
			l = corners[6][1]
			t = corners[2][2]
			r = corners[2][1]
			b = corners[6][2]
		else
			l = corners[8][1]
			t = corners[4][2]
			r = corners[4][1]
			b = corners[8][2]
		end
	elseif corcode == 'QECcCZzZ' then
		print('unused octrect '..left..','..top..': '..corcode)
	elseif corcode == 'QEeEeECcCcCZzZzZQqQq' or
	corcode == 'QqzZzcCceEeq' then
		-- screw attack
	elseif corcode == 'QEeEeECZzZ' then
		-- grapple
		
	elseif corcode == 'QECcCcCZQq' then
		-- grapple
		
	elseif corcode == 'QECcCZQqQq' then
		-- grapple
		
	elseif corcode == 'QEeECZzZzZ' then
		-- grapple
		
	else
		print('unknown shape '..left..','..top..': '..corcode)
	end
	if l and t and r and b then
		split_frame(l+1, t+1, r-l, b-t)
	end
	return corners[2][1] - left
end

-- This while loop goes through the entire sheet, just looking for upper_left corners.
-- If it finds one that isn't already known part of a frame, it handles that frame.
local x, y = 0, 0
while y < src_h-1 do
	x = 0
	while x < src_w-1 do
		if corner_names[get_corner_type(x, y)] == 'upper_left'
		and not used_corners[x+(y*src_w)] then
			local mv_x = handle_frame(x, y)
			if not mv_x then
				x = src_w
			else
				x = x + mv_x
			end
		else
			x = x + 1
		end
	end
	y = y + 1
end

-- replace pngdump:Dump with whatever writes the dest sheet to a file.
-- just gives the new file the same name as the old with "_split.png" appended.
local dest_file = io.open(passed_path:sub(1, -5)..'_split.png', 'wb')
pngdump:Dump(dest, dest_file, dest_file.write)